Low-Q 2 low-x Structure Function Analysis of CCFR data for F 2

Transcription

1 Low-Q low-x Structure Function Analysis of data for F B. H. Tamminga a,t. Adams b,a.alton b,c.g.arroyo a,s.avvakumov c, L. de Barbaro d,p.debarbaro c,a.o.bazarko a,r.h.bernstein e,a.bodek c, T. Bolton b,j.brau f, D. Buchholz d, H. Budd c,l.bugel e,j.conrad a,r.b.drucker f, J. A. Formaggio a,r.frey f, J. Goldman b, M. Goncharov b,d.a.harris c,r.a.johnson g,j.h.kim a,b.j.king a,t.kinnel h, S. Koutsoliotas a,m.j.lamm e,w.marsh e, D. Mason f,k.s.mcfarland c,c.mcnulty a,s.r.mishra a, D. Naples b,p.nienaber e, A. Romosan a,w.k.sakumoto c, H. Schellman d,f.j.sciulli a,w.g.seligman a, M. H. Shaevitz a,w.h.smith h,p.spentzouris a,e.g.stern a, M. Vakili g, A. Vaitaitis a, V. Wu g,u.k.yang c,j.yu e,g.p.zeller d,e.d.zimmerman a a Columbia University, New York, NY 7 b Kansas State University, Manhattan, KS 6656 c University of Rochester, Rochester, NY 467 d Northwestern University, Evanston, IL 68 e Fermi National Accelerator Laboratory, Batavia, IL 65 f University of Oregon, Eugene, OR 9743 g University of Cincinnati, Cincinnati, OH 45 h University of Wisconsin, Madison, WI 5376 Analyses of structure functions (SFs) from neutrino and muon deep inelastic scattering (DIS) data have shown discrepancies in F for x<.. A new SF analysis of the collaboration data examining regions in x down to x =.5 and.4 <Q <.is presented. Comparison to corrected charged lepton scattering results for F from the and experiments are made. Differences between µ and ν scattering allow that the behavior of F µ could be different from F ν as Q approaches zero. Comparisons between F µ and F ν are made in this limit. High-energy neutrinos are a unique probe for understanding the parton properties of nucleon structure. Combinations of ν and ν DIS data are used to determine the F and xf 3 SFs which determine the valence, sea, and gluon parton distributions in the nucleon [,]. The universalities of parton distributions can also be studied by comparing neutrino and charged lepton scattering data. Past measurements have indicated that F ν differs from F e/µ by -5% in the low-x region [3]. These differences are larger than the quoted combined statistical and systematic errors of the measurements and may indicate the need for modifications of the theoretical modeling to include higher-order or new physics

2 contributions. We present a new analysis of the collaboration ν-n DISdataina previously unexplored kinematic region. In this low-x and low-q region, the discrepancy between F ν and F µ persists. However, in this kinematic region some differences in F from neutrino and charged lepton data may result from differences in the properties of weak and electromagnetic interactions. Within the PCAC nature of ν-n DIS, F ν should approach a constant as Q approaches zero, while F e/µ for charged lepton DIS should approach zero. A determination of this constant is presented. The ν DIS data were taken in two high-energy high-statistics runs, FNAL E744 and E77, in the Fermilab Tevatron fixed-target quadrupole triplet beam (QTB) line by the collaboration. The detector, described in Refs. [4,5], consists of a target calorimeter instrumented with both scintillators and drift chambers for measuring the energy of the hadron shower E HAD and the µ angle θ µ, followed by a toroid spectrometer for measuring the µ momentum p µ. There are 95, ν µ events and 7, ν µ events in the data sample after fiducial-volume cuts, geometric cuts, and kinematic cuts of p µ > 5 GeV, θ µ < 5 mr, E HAD > GeV,and3<E ν <36 GeV, to select regions of high efficiency and small systematic errors in reconstruction. In order to extract the SFs from the number of observed ν µ and ν µ events, determination of the flux was neccesary [7,6,8]. The cross-sections, multiplied by the flux, are compared to the observed number of ν-n and ν-n events in each x and Q bin to extract F (x, Q ) and xf 3 (x, Q ). Determination of muon and hadron energy calibrations from the previous analysis were used in the present analysis. These calibrations were determined from test beam data collected during the course of the experiment [4,5]. Changes in the SF extraction to extend the analysis into the low-q,low-xregion include incorporation of an appropriate model below Q of.35 GeV, in this case we chose the GRV [] model of PDFs. The data have been corrected using the leading order Buras-Gaemers model [9] for slow rescaling [,3], with charm mass of.3 GeV and for the difference in xf3 ν xf 3 ν. In addition, corrections for radiative effects [], non-isoscalarity of the Fe target, and the mass of the W -boson propagator were applied. Due to the systematic uncertainty in the model, the radiative correction error dominates in the lowest x bins. Other significant systematics across the entire kinematic region include the value of R, whichcomesfrom a global fit to the world s measurements []. The SF F from ν DISonironcanbecomparedtoF from charged lepton DIS on isoscalar targets. To make this comparison, two corrections must be made to the charged lepton data. For deuterium data, a heavy nuclear target correction must be made to convert F ld to F lfe [4]. Second, a correction was made to account for the different quark charge involved in the charged lepton DIS interactions [3]. The errors on the nuclear and charge corrections are small compared to the statistical and systematic errors on both the and data. The corrected SF, F, from µ DIS experiments and [5,6] along with for lowest x-bins is shown in Fig.. The new analysis allows comparison to data, which is in the low-x, low-q region. Error bars for and data are large in the x-bin, x=.5. However, In the next x-bin, x =.45, there is clearly as much as a % discrepancy between the F µ and the F ν and an approximately % discrepancy between and. As the value of x increases, the discrepancy decreases; there is agreement between and the charged lepton experiments above x =..

3 3 F.5 x=.5 (x=.9) (x=.) (x=.8) (x=.5) x=.75.5 x=.45 x=.5 F F F.5.5 x=.8 x=.5 (x=.37) (x=.5) (x=.69) (x=.89) x=.35 x=.5 Q Q Figure. F from ν-fe DIS compared to F from µd DIS. Errors bars are statistical and systematic added in quadrature. The µ data have been corrected as described in the text. The discrepancy between and at low-x is outside the experimental systematic errors quoted by the groups. Several suggestions for an explanation have been put forward. One suggestion [7], that the discrepancy can be entirely explained by a large strange sea, is excluded by the dimuon analysis which directly measures the strange sea [8]. Another is that the strange sea may not be the same as the anti-strange sea distribution. Data from both and do not support this possibility [9]. Another possibilty is that the heavy nuclear target correction may be different between neutrinos and charged leptons. Heavy target corrections used in this paper are determined by for charged lepton-nucleon DIS data and applied to and only; no charged lepton correction data is applied to ν data. Another possibility that has been proposed would have a large symmetry violation in the sea quark [], but recently the model has been ruled out by the CDF W charge asymmetry measurements []. Finally, in the low-x and low-q region, some of the discrepancy may be accounted for by the differences in behavior of F as Q approaches zero, although this can only address the x<.75 region. In charged lepton DIS, the SF, F, is constrained by gauge invariance to vanish linearly with Q at Q =. Donnachie and Landshoff predict that in the low-q region, F µ will follow the form [] C ( ) Q Q +A. However, in the case of neutrino DIS, the PCAC nature of the weak interaction contributes a nonzero component to F as Q approaches zero. Donnachie and Landshoff predict that F ν contribution at Q =: C ( ) should follow a form with a non-zero Q + Q +D Q +A Q +B. Using data we fit to the form predicted

4 4 Table Fit results for and data. is fit to Eq. 3 and parameter A extracted: A =. ±.7. data is fit to Eq. 4 with A extracted from fit. B, C, D and F at Q results shown below. x B C D F ν (Q =) χ ±.3.57 ±.9.4 ±.3. ± ±.4.34 ±.6.64 ±.6.33 ± ±.4.8 ±.5.7 ±.6.36 ± ±.4.3 ±.5.64 ±.6.33 ±.4.8 for e/µ DIS, extracting the parameter A. Inserting this value for A into the form predicted for ν DIS, we fit data to extract parameters B,C,D, and determine the value of F at Q =. OnlydatabelowQ =.35 GeV are used in the fits. The x-bins having enough data for a good fit in this Q region are x =.45, x =.8, x =.5, x =.75. Table shows the results of the fits. The values of F at Q = in the three highest x-bins are statistically significant and in agreement with each other. The lowest x-bin is consistent with the other results. In summary, a comparison of F from ν DIS to that from µ DIS continues to show good agreement above x =. but a difference at smaller x that grows to % at x =.45. The experimental systematic errors between the two experiments, and improved theoretical analyses of massive charm production in both neutrino and muon scattering are both presently being investigated as possible reasons for this discrepancy. Some of this low-x discrepancy may be explained by the different behavior of F from ν DIS to that from e/µ DIS at Q =. F ν data appear to approach a non-zero constant at Q =. REFERENCES. M. Glück, E. Reya, A. Vogt, Z. Phys. C67: 433 (995); A. D. Martin, R. G. Roberts, W. J. Stirling; DTP/96/44, RAL-TR (996).. H. L. Lai et al., Phys.Rev. D55: 8 (997). 3. W. G. Seligman et al., Phys. Rev. Lett. 79: 3 (997). 4. W. K. Sakumoto et al., Nucl. Instrum. Meth. A94: 79 (99). 5. B. J. King et al., Nucl. Instrum. Meth. A3: 54 (99). 6. P. S. Auchincloss et al., Z. Phys. C48: 4 (99). 7. W. G. Seligman, Ph. D. Thesis, Nevis Report R. Belusevic and D. Rein, Phys. Rev. D38: 753 (988). 9. A. J. Buras and K. J. F. Gaemers, Nucl. Phys. B3: 49 (978). D. Yu. Bardin, V. A. Dokuchaeva, JINR-E-86-6 (986).. L. W. Whitlow et al., Phys. Lett. B5: 93 (99).. R. M. Barnett, Phys. Rev. Lett. 36: 63 (976). 3. H. Georgi and H. D. Politzer, Phys. Rev. D4: 89 (976). 4. P. Amaudruz et al., Nucl. Phys. B44: 3 (995); R. G. Arnold et al., Phys. Rev. Lett.

5 5: 77 (984); J. Gomez et al., Phys. Rev. D49: 4348 (994); M. R. Adams et al., Z. Phys. C67: 43 (995). 5. M. Arneodo et al., Nucl. Phys. B483: 3 (997). 6. M.R. Adams et al., Phys.Rev. D54: 36 (996); L. W. Whitlow, Ph.D. Thesis, SLAC-REPORT-357 (99); A.C. Benvenuti et al., Phys. Lett. B37: 59 (99). 7. J. Botts et al., Phys. Lett. B34: 59 (993). 8. A. O. Bazarko et al., Z. Phys. 65: 89 (995). 9. S. Brodsky and B. Ma, Phys. Lett. B38: 37 (996).. C. Boros, J.T. Londergan, and A.W. Thomas, Phys. Rev. Lett. 8, 475, (998); also ADP-98-64/T33 (hep-ph-98). A. Bodek et al., UR-657,hep-ex/994, to be published in Phys. Rev. Lett.. A. Donnachie and P.V. Landshoff Z. Phys. C6: 39 (994) 5